1. Field of the Invention
This invention relates to a method for the production of a hydrophilic resin having an acrylate as a main component thereof. More particularly it relates to a method for the production of a hydrophilic resin which has only a small residual monomer content and shows virtually no increase in the residual monomer content under varying conditions of use.
The hydrophilic resin according to this invention can be produced easily and inexpensively and is excellent in quality and safety and, therefore, can be used as an absorbent resin and water-soluble resin in a wide range of applications.
2. Description of the Prior Art
The hydrophilic resins can be generally classified by their solubility in water into roughly two types, water-soluble resins and absorbent resins.
Water-soluble resins are hydrophilic resins of the type which dissolve in water and are used, for example, as water treatment grade flocculants, oil drilling additives, food additives, and viscosity enhancers.
The water-soluble resins which are known to the art include, for example, polysodium acrylate (JP-B-48-42,466 and JP-B-42-9,656), polyacrylic acid and polyacrylamide (JP-A-54-145,782 and JP-A-57-18,652), polymers of 2-acrylamide-2-methylpropane sulfonic acid (JP-A-2-173,108), partial hydrolyzate of polyacrylamide (JP-A-52-137,483), acrylic acid-acrylamide copolymer (JP-A-59-15,417), (meth)acrylic acid-itaconic acid copolymer (JP-A-58-91,709), and polyvinyl alcohol.
Absorbent resins are water-insoluble hydrophilic resins of the type which absorb water and consequently undergo gelation and are widely used in the fields of agriculture and forestry and in the field of civil engineering as well as in the field of hygienic materials such as disposable diapers and sanitary napkins.
The absorbent resins which have been heretofore known include, for example, partially neutralized cross-linked polyacrylic acid (JP-A-55-84,304, JP-A-55-108,407, and JP-A-55-133,413), hydrolyzate of starch-acrylonitrile graft polymer (JP-A-46-43,995), neutralized starch-acrylic acid graft polymer (JP-A-51-125,468), saponified vinyl acetate-acrylic ester copolymer (JP-A-52-14,689), hydrolyzate of acrylonitrile copolymer or acrylamide copolymer (JP-A-53-15,959) or cross-linked derivatives thereof, and cross-linked cationic monomer (JP-A-58-154,709 and JP-A-58-154, 710).
Numerous compounds have been proposed as monomers for the production of these hydrophilic resins. From the viewpoint of the quality of the product and the cost of production, a partially or completely neutralized acrylate (hereinafter referred to as “acrylate”) is predominately used today. The acrylate type polymers which have acrylates as the main component of their monomers are produced today in large amounts for both absorbent resins and water-soluble resins and have been finding extensive utility in the fields of hygienic materials and foodstuffs.
By the current technical standard, it is normal that the acrylate type polymers which are, in wide use generally contain such a residual monomer as unaltered acrylic acid (or a salt thereof) in a concentration in the range of 500 to 3,000 ppm. Thus, the desirability of decreasing the residual monomer content in the polymers has been finding enthusiastic recognition.
In these hydrophilic resins, particularly the absorbent resins used in sanitary materials, a lower the residual monomer content is required. In recent years, the prevailing demand is to lower the residual monomer content to below 100 ppm. This decrease of the residual monomer content is particularly difficult to attain in the absorbent resins among the hydrophilic resins because the absorbent resins are hydrophilic resins of the type having a cross-linked structure and, therefore, more often than not have a neutral pH value.
The absorbent resins having a cross-linked structure are not easily polymerized uniformly as compared with water-soluble resins. When absorbent resins fresh from polymerization are to be mixed with an additive to decrease the residual monomer content or with an organic solvent, a uniform mixture is not easily obtained because of the cross-linked structure of the polymers. Thus, a decrease of the residual monomer content in the absorbent resins has been extremely difficult to attain. Moreover, since acrylates are such that their polymerization velocities are lowered proportionately as their pH values approach neutrality, a decrease of the residual monomer content as a consequence of polymerization has been extremely difficult to attain in neutral acrylate type absorbent resins.
Heretofore, in the field of macromolecular flocculants, for example, there has been an attempt at decreasing the residual monomer content in hydrophilic resins. Even now, numerous hydro-philic resins mentioned above as well as acrylate type polymers and absorbent resins are still the subjects of a study in a search for measures of decreasing the residual monomer content.
The techniques known to the art are broadly divided into the following six types (a) to (f):                (a) Methods for lowering the residual monomer content by increasing the polymerization ratio of the polymer itself.        
The methods of this type include, for example, increase of the amount of polymerization initiator and the use of a composite initiator (JP-A-50-96,689), elevation of the reaction temperature, an increase in the polymerization concentration, lengthening the polymerization time, and the specification of aging conditions (JP-A-53-145,895), the two-stage addition of a polymerization initiator (JP-A-56-72,005), the exposure of a resin fresh from polymerization to radiation (JP-A-63-43,930), and the irradiation of a resin fresh from polymerization with ultraviolet light (JP-A-62-260,906).                (b) Methods for converting the residual monomer in the polymer into an extraneous derivative by use of an additive.        
The methods of this type include, for example, the subsequent addition of a primary or secondary amine (JP-A-50-40,649), the subsequent addition of sulfur dioxide (U.S. Pat. No. 3,780,006), and the subsequent addition of an alkali metabisulfite (U.S. Pat. No. 4,306,955).                (c) Methods for extracting the residual monomer from the polymer.        
The methods of this type include, for example, the extraction by the use of a hydrophilic organic solvent (U.S. Pat. No. 4,794,116) and the supercritical extraction by the use of carbon dioxide.                (d) Methods for treating the residual monomer with a micro-organism capable of decomposing the residual monomer        
The methods of this type include, for example, decomposition of residual acrylamide with a microorganism (U.S. Pat. No. 4,742,114).                (e) Methods for volatilizing the residual monomer at elevated temperatures        
The methods of this type include, for example, volatilization of residual acrylonitrile at an elevated temperature (JP-A-54-119,588).
The methods of (a), however, are actually such that since their effects in lowering the residual monomer content are not sufficient, the residual monomer generally persists in a concentration of at least 0.03% and the self-crosslinking occurs and basic molecular weight of the hydrophilic resin are necessarily degraded by the harsh conditions during polymerization and the aftertreatment possibly to the extent of increasing the water-soluble content of the absorbent resin, lowering the gel strength, and impairing the physical properties of the resulting hydrophilic resin.
Further, the two-stage addition of a polymerization initiator and the use of a large amount of initiator increases the possibility of the polymerization initiator persisting in the produced resin and consequently jeopardizing the safety of the produced polymer.
The methods of (b) and (c) are purportedly capable of lowering the residual monomer content to below 0.03%. However in (b), in addition to complexed process, an additive used and the adduct formed of the additive with the residual monomer and in (c) the organic solvent such as methanol used for the extraction of residual monomer, never fail to persist in the hydrophilic resin.
The effect of the method of (c) in lowering the residual monomer content in the resin, however, is limited because acrylates are not dissolved in such organic solvents as methanol.
The method of (d) is not easily carried out on a commercial scale because of the use of a microorganism. Moreover, the use of the microorganism itself proves to be undesirable from the standpoint of safety.
The method of (e) is observed at times to impair various physical properties by the elevated temperatures. Moreover, since the acrylate fails to volatilize even at elevated temperatures, this method can hardly be expected to attain an effective decrease of the residual monomer content.
More recently, (f) the efforts directed to the reduction of the residual monomer content have revealed in the water-soluble unsaturated monomer as a matter deserving due attention before polymerization. The method embodying this knowledge has also been known to the art.
The methods of this type already known to the art include polymerization effected by the use of an acrylate obtained by a specific method of neutralization (EP-A-0372706) and polymeri-zation attained by the use of a monomer having a small heavy metal content (JP-A-3-31,306), for example.
The methods of the type of (f), however, are not sufficiently effective in lowering the residual monomer content.
The methods of the various types cited above, however, are actually such that they not only fail to produce the required effect but also necessitate a complicated process and involve a sacrifice in productivity and physical properties and a large addition to the cost of production. Moreover, these methods only bring about an apparent decrease in the residual monomer content in the acrylate type polymer and are totally incapable of repressing the increase of the residual monomer content which occurs subsequent to the polymerization as described hereinbelow.
We have found that in the acrylate polymer which is obtained by the conventional method, an unaltered monomer persists in such high concentrations as to fall in the approximate range of some tens of ppm to some thousands of ppm in addition to the residual monomer of the type generally known in the art. We have also found that even when the residual monomer content of the acrylate polymer is decreased apparently to a level of some hundreds of ppm, the residual monomer actually increases proportionately over a period of time. This increase over a period of time is particularly conspicuous when the polymer is heated.
When the acrylate polymer having an apparently small residual monomer content is required to undergo further heating or when it is used in an agronomic field for a long time or exposed to an elevated temperature such as, for example, hot water, it is natural to conclude that this use of the polymer is undesirable from the viewpoint of safety because the residual monomer content is increased under such conditions of use.
Concerning the production of the acrylate polymer, many techniques have been known to cross-link the surface region of the acrylate polymer for the purpose of improving the various physical properties thereof besides the mere operations of polymerization and drying. Particularly in the field of absorbent resins, various surface cross-linking agents and reaction conditions for surface cross-linking have been proposed because cross-linking near the surface region exerts numerous effects on the physical properties of the absorbent resin.
The methods for surface cross-linking the acrylate polymer by the use of specific surface cross-linking agents heretofore known to the art include, for example, a method using a polyhydric alcohol (JP-A-58-180,233 and JP-A-61-16,903), a method using an alkylene carbonate (DE-4020780C), a method using glyoxal (JP-A-52-117,393), a method using a polyvalent metal (JP-A-51-136,588, JP-A-61-257,235, and JP-A-62-7,745), and a method using a silane coupling agent (JP-A-61-211,305, JP-A-61-252,212, and TP-A-61-264,006). The methods for surface cross-linking the resin under specific reaction conditions which have been heretofore known to the art include, for example, a method effecting the desired cross-linking by dispersing an absorbent resin in a mixed solvent consisting of water and a hydrophilic organic solvent (JP-A-57-44,617), a method effecting the cross-linking by dispersing an absorbent resin in an insert medium in the presence of a specific amount of water (JP-A-58-117,222), a method effecting the cross-linking by establishing co-existence of an inorganic powder and water(U.S. Pat. No. 4,587,308), and a method resorting to exposure of the polymer to an electromagnetic radiation (JP-A-63-43,930).
The various methods thus proposed, however, are invariably incapable of sufficiently improving the various physical properties of the acrylate polymer by surface cross-linking. Thus, studies are still continuing to attain thorough improvement of the physical properties.
In the process of studying an improvement of the surface cross-linking of the acrylate polymer, we have found the heretofore totally unknown fact that surface cross-linking markedly increases the residual monomer content in the acrylate polymer to a level in the approximate range of some tens of ppm to some hundreds of ppm and that the increase of the residual monomer content due to surface cross-linking accounts for a large proportion of the residual monomer content in the final product.
In a hydrophilic resin of an acrylate polymer which does not easily allow a decrease of the residual monomer content and has the peculiar phenomenon of newly generating residual monomer or increasing the amount of already existent residual monomer in the polymer during manufacture of the resin or during use of the resin, this invention has an object of providing a method for the production of a hydrophilic resin having excellent physical properties, having only a small residual monomer content, and showing virtually no sign of generation or augmentation of residual monomer after polymerization.
In a hydrophilic resin of an acrylate polymer which has various physical properties improved by cross-linking the surface region thereof, this invention has another object of providing a method for the production of a hydrophilic resin which has the surface region thereof cross-linked and is consequently able to have a noticeable effect in improving various physical properties and enjoys a notable decrease in the residual monomer content thereof.